The invention relates to a photoconductive assembly which is capable of transferring developed toner images to a receptor.
Photographic microfilm, i.e. microfilm employing silver halide for image formation, is capable of providing resolution in the range of about 200 to 400 line pairs/mm. Although this degree of resolution is acceptable, microfiche prepared from photographic microfilm is impossible to update, i.e. add additional images at a date subsequent to development, because the development system results in destruction of the light-sensitivity of the microfiche.
Updatable microfiche comprises a photoconductive sheet, upon which sheet microimages are toner-developed by means of electrostatic processes. These photoconductive sheets suffer from two primary drawbacks: (1) they generally have a colored background, which results in poor contrast; (2) over time, the characteristics of the photoconductive sheet change, rendering it unacceptable for further updating.
Accordingly, it would be desirable to transfer microimages from the photoconductive sheet upon which they are developed by electrostatic processes to an inert microfiche sheet that would provide good contrast and not degrade over time, so that the microfiche would be updatable even after a long period of time, e.g., several years.
As is well-known in the art, the basic electrostatic process involves placing a uniform electrostatic charge on a photoconductive insulating layer, imagewise exposing the layer to dissipate the charge on the areas of the layer exposed to light, and developing the resulting electrostatic latent image with a material known as toner. The toner will normally be attracted to those areas of the layer which retain a charge, thereby forming a toner image corresponding to the electrostatic latent image. The toner image can then be transferred to a support surface such as a polymeric film. Toner can be either a powdered material comprising a blend of polymer and carbon or liquid material comprising an insulating liquid vehicle having finely divided solid material dispersed therein. Toner-developed microimages are preferably prepared with liquid toner developer, rather than powdered toner developer, because liquid toner developers are capable of giving higher resolution images with better gradation than powdered toner developers.
A problem that arises with the employment of liquid toner developers, however, is poor transfer from the photoconductor to the receptor, particularly when transfer is effected by heat, pressure, or a combination of heat and pressure. It is desirable to employ a combination of heat and pressure to effect transfer in order to improve gray scale fidelity. Poor transfer is manifested by (a) low transfer efficiency, e.g., below about 50 percent, and (b) low image resolution, e.g. less than about 80 line pairs/mm. Low transfer efficiency results in images that are light and/or speckled. Low image resolution results in images that are fuzzy.
Currently, the efficiency of toner transfer, when transfer is effected by means of heat and pressure, after any standard liquid development, generally cannot be raised to a high level, e.g. in excess of about 50 percent, without substantial loss in resolution. Furthermore, the level of resolution is generally limited to levels of up to 80 line pairs/mm.